This is a revised application for Project by Letendre. This Project will use a translational approach to understand the effects of METH, HIV, and HCV on the brain by using in vivo biomarkers as windows into cellular and molecular mechanisms of neuropathogenesis. We will examine six categories of biomarkers (astroglial, inflammatory, oxidative, trophic, neuronal, and viral), which correspond to the major pathogenic mechanisms outlined in our conceptual model. The specific aims will be to identify distinct biomarker profiles that are associated with METH, HIV, and HCV;to identify the relationships between METH and HIV and two specific groups of biomarkers, fibroblast growth factors (FGFs) and interferon-inducible proteins;and to determine the relationships between biomarker profiles and key indicators of brain injury being studied in other Projects, including neurocognitive impairment, movement disorders, altered regional brain activation, and changes in glutamine-glutamate cycling. To accomplish these aims, we will examine biomarker concentrations in CSF from 425 participants from the three risk groups enrolled into this Program. We will also examine changes in biomarker profiles and CSF concentrations of interferon (IFN)-alpha in response to PEG-IFN-alpha/ribavirin treatment of HCV infection in METH+ individuals enrolled in Study 2 of Project by Heaton. In order to more specifically determine the effects of recent versus long-term abstinence from METH on biomarker profiles, an added group of 50 METH users who have used within 10 days will be enrolled specifically for this Project. These experiments are innovative because they will determine the unique biomarker signatures of METH, HIV, and HCV;measure CSF markers reflecting multiple mechanisms of injury in the same specimens;investigate recently recognized mechanisms of injury, such as FGFs;link in vivo and in vitro findings;and determine the effects of PEG-IFN-alpha and ribavirin on putative in vivo markers of neuropathogenesis.